The invention relates to a method for determining the composition of a fuel mixture consisting of a first and a second fuel for operating an internal combustion engine having at least one cylinder pressure sensor for determining the cylinder pressure in at least one cylinder of the internal combustion engine.
The invention further relates to a device for determining the composition of a fuel mixture consisting of a first fuel and a second fuel for operating an internal combustion engine, wherein the internal combustion engine has at least one cylinder pressure sensor for determining the cylinder pressure in at least one cylinder of the internal combustion engine. Said device comprises a control unit for controlling the internal combustion engine, for determining an indicated mean effective pressure pmi and an indicated work Wi during the high pressure loop of a work cycle from the signals of the cylinder pressure sensor and for determining the injected fuel quantity.
Internal combustion engines on the basis of Otto engines are generally operated with fuel from hydrocarbons from fossil fuels based on refined crude oil. Alcohol produced from renewable resources (plants), for example ethanol or methanol, is increasingly being added in various mixing ratios to this fuel. Depending on the market, the ethanol proportion in fuel mixtures lies in a wide range from 0% to 85%. An operation with pure ethanol is likewise conceivable (Brazil). In the USA and Europe a mixture of 75-85% ethanol and 15-25% gasoline is often distributed under the trade name E85. By adding pure gasoline, all fuel compositions under an ethanol proportion of 85% can thereby result.
The internal combustion engines are designed in such a way that they can be operated with pure gasoline as well as with mixtures up to E85 without serious restrictions and retrofittings. The operating parameters of the internal combustion engine have to be adapted to the respectively existing fuel mixture for an efficient operation with only a small discharge of toxic emissions to occur while a high degree of engine performance is maintained at the same time. A stoichiometric air-fuel mixture ratio is by way of example present at 14.7 volumetric parts of air per part of gasoline; however, when using ethanol, a proportion of air of 9 volumetric parts must be set. For this purpose, it is known how to determine a corresponding adaptation or correction value with the aid of the so-called mixture adaptation, which is based on the signal of a lambda or oxygen probe in the exhaust gas tract of the internal combustion engine, and thus how to adapt the air/fuel ratio to the fuel composition. This requires however a very large adjusting range of the mixture adaptation. Only limited correction options are then often available for shifts in lambda resulting from tolerances in the fuel path (fuel pressure error, flow error of the injection valves) or in the air path (air mass error resulting from tolerances of the hot-film air-mass meter (HFM) or from pressure sensors).
On account the different burning behavior of ethanol and gasoline, the necessity further arises to perform a control of the ignition of the internal combustion engine which is independent of the ethanol content of the fuel mixture.
It is therefore known how to determine the composition of the fuel mixture using separate sensors and correspondingly how to take this information into account in the engine management system. For this purpose, different fuel type sensors, also denoted as “fuel composition sensors”, are used. Fuel composition sensors use the different properties of alcohol and gasoline to determine the fuel composition. Ethanol is then, for example, a protic solvent which contains hydrogen ions and has a large permittivity, albeit dependent on the water content. Gasoline on the other hand is an aprotic solvent with a small permittivity. Based on this fact, there are fuel composition sensors, which determine the fuel composition using the dielectric properties of the fuel mixture. Other fuel composition sensors use the different optical characteristics of the fuels, for example the different refractive indices. The use of additional sensors entails however increased costs.
In order to optimize the ignition, alternative methods are known, in which thermodynamic auxiliary quantities, which are formed with the aid of the signals of cylinder pressure sensors, are adjusted to predetermined nominal values. The ignition can thereby be set to optimal values without explicit knowledge of the composition of the fuel mixture.
The German patent publication DE 10 2007 060 223 A1 describes a method for determining the composition of a fuel mixture consisting of a first fuel and a second fuel or for determining the quality of a fuel for operating an internal combustion engine having at least one cylinder pressure sensor in at least one cylinder of the internal combustion engine for determining the pressure course during a combustion stroke and a cylinder pressure based engine regulation for regulating the load and combustion position of said internal combustion engine. It is thereby provided that the determination of the composition of the fuel mixture or the quality of the fuel takes place with the aid of regulating information of the cylinder pressure based engine regulation. The invention proposes that the determination of the composition of the fuel mixture or the quality of the fuel takes place on the basis of manipulated variable corrections and/or regulation deviations of the cylinder pressure based engine regulation.
The German patent publication DE 10 2007 023 900 A1 describes a method for determining the composition of a fuel mixture consisting of a first fuel and a second fuel for operating an internal combustion engine, wherein the first and the second fuel provide different rates of combustion and/or different specific energy contents and wherein the internal combustion engine provides at least one pressure sensor in at least one combustion chamber, with which a temporal and/or angle synchronic pressure course is determined in the combustion chamber. Provision is thereby made for the composition of the fuel mixture to be determined from the temporal and/or angle synchronic pressure course of the gas pressure in the at least one combustion chamber during a combustion phase.